This invention relates to the manufacture of pigmented organic vehicles. In particular, it relates to an improved process for conditioning colored organic pigment by partial comminution of the crude pigment in an attrition, shear or impact type mill followed by a further conditioning of the pigment by treating the partially milled pigment with a breaching agent during a flushing operation to produce a pigmented organic vehicle having a pigment of enhanced pigmentary quality.
Many of thee colored pigments in commercial use at the present time are crystalline organic compounds which have been synthesized by chemical reaction. These compounds in their crude state are often relatively large as pigment particles, and satisfactory pigmentary properties are obtained only after the crude pigments have been subjected to particle size reduction and/or phase changes in the crystalline structure. For most pigments a particle size range of about 0.05 to 0.1 micron is desirable. Most commercial pigments have a particle size well below 0.2 microns, and the crude pigments, as prepared by most synthetic organic processes, have an average particle size of at least 5 to 20 microns. The desired reduction in particle size can often be achieved by prior art grinding processes alone; however, the amount of energy required and low production due to long grinding time has led workers in this art to seek shorter manufacturing methods for high quality pigments.
Numerous attempts have been made to grind pigments in the dry form without the addition of grinding aids; however, it has been found very difficult to produce satisfactory pigmentary particles. Common practice in industrial manufacture of organic pigments such as phthalocyanines is to charge an attrition-type mill with a large amount of inorganic salt in addition to the grinding elements. Typically, the prior art processes employed about 9 parts sodium chloride or borax per part of pigment when grinding in the dry state. With the total volume of grinding elements and salt being so large, only a relatively small amount of pigment could be milled for each batch.
It has been found that a subsequent treatment of dry milled pigment by agitation with a suitable organic liquid can improve the pigmentary properties. In U.S. Pat. Nos. 2,857,400 and 3,017,414 it has been demonstrated that partial milling of a crude pigment in the dry state, as by ball milling in the absence of any grinding aid, when treated with an organic liquid or aqueous emulsion of certain organic liquids can produce a satisfactory pigment. Typical organic liquids suitable for this treatment include dimethyl formamide, tetrachloroethylene, carbon tetrachloride,, o-dichlorobenzene, chlorobenzene, benzene, xylene, nitrobenzene, and benzonitrile. In British Pat. No. 1,087,004, the organic breaching liquids used for treating partially milled organic pigments included chloroform and 1,2-dibromo-1,1,2,2-tetrafluoroethane among the halogenated hydrocarbons suitable. Hexane and cyclohexane were found to be suitable parafinic and cycloaliphatic hydrocarbons. It was established that about 0.1 to 3 parts organic breaching liquid per part pigment was satisfactory to provide the essential characteristic of the breaching liquid in its apparent ability to wet the pigment surface and to diffuse along grain boundaries and into interstitial areas of pigment aggregates.
In the manufacture of most organic pigments, it is usually necessary to finish the pigment in several physical forms. In some cases the pigment is ground into the vehicle with which it is to be used. In other cases the use of toners, or dry undiluted power, is desirable. Diluted dry powders, or "lakes", undispersed presscakes, dispersed pastes and water-dispersible powders are other common forms of organic pigments offered commercially. In the prior art partial milling and breaching processes, it has been found necessary to recover the pigment from the breaching mixture, usually by distillation or other evaporation step, followed by some other finishing step to render the pigment commercially acceptable. Where the pigments are made in the form of toners or undiluted powders, the conversion of a wet pigment into dry powder form is not a straightforward matter. Most wet organic pigments tend to agglomerate irreversibly on drying, yielding hard, gritty powders of low tinctorial strength. In some manufacturing processes the addition of small amounts of fatty acids or wetting agents prior to drying can help solve this agglomeration problem. In the salt-milled pigments, which produce wet presscakes which may be dried directly to powder form, various finishing steps may be required to produce a satisfactory product. After the pigment powder has been dried, tinctorial strength may be lost during final pulverization or any amount of overgrinding may prove detrimental.
One procedure for utilizing wet or dispersed presscakes of conditioned organic pigments is the flushing process. This method makes possible the transfer of hydrophobic organic particles from an aqueous phase into an organic vehicle. This method has been used commercially to by-pass recovery of the fully conditioned pigment, thereby preventing possible loss of tinctorial strength due to drying or pulverization. The flushing process itself disperses the pigment thoroughly in the organic vehicle without further grinding or milling of the pigmented organic phase. The typical organic pigments which are conditioned by attrition milling and/or breaching are usually more readily wetted by an organic liquid phase or soft oleoresinous vehicle than by water; therefore, an aqueous slurry of such pigment is mixed with the organic vehicle and is transferred to the vehicle during the flushing process. The major amount of water is removed by conventional mechanical separation procedures.